When it’s a close vote and circumstances are changing rapidly, I’d say pretty often. Maybe every two or three years until some kind of consensus emerges? It seems like an important topic you wouldn’t want to suppress except for once every sixty years or whatever, and might force some negotiation to get to a better place whether through independence or reconciliation.

This is assuming the role of the government here is to track what the people want and execute their will. Maybe it’s different in the UK (again, I don’t live there and I know it’s a complicated system).